Abstract
Large-scale modifications to urban underlying surfaces owing to rapid urbanization have led to stronger urban heat island (UHI) effects and more frequent urban heat wave (HW) events. Based on observations of automatic weather stations in Beijing during the summers of 2014–2020, we studied the interaction between HW events and the UHI effect. Results showed that the UHI intensity (UHII) was significantly aggravated (by 0.55°C) during HW periods compared to non-heat wave (NHW) periods. Considering the strong impact of unfavorable weather conditions and altered land use on the urban thermal environment, we evaluated the modulation of HW events and the UHI effect by wind speed and local climatic zones (LCZs). Wind speeds in urban areas were weakened due to the obstruction of dense high-rise buildings, which favored the occurrence of HW events. In detail, 35 HW events occurred over the LCZ1 of a dense high-rise building area under low wind speed conditions, which was much higher than that in other LCZ types and under high wind speed conditions (< 30 HW events). The latent heat flux in rural areas has increased more due to the presence of sufficient water availability and more vegetation, while the increase in heat flux in urban areas is mainly in the form of sensible heat flux, resulting in stronger UHI effect during HW periods. Compared to NHW periods, lower boundary layer and wind speed in the HW events weakened the convective mixing of air, further expanding the temperature gap between urban and rural areas. Note that LCZP type with its high-density vegetation and water bodies in the urban park area generally exhibited, was found to have a mitigating effect on the UHI, whilst at the same time increasing the frequency and duration of HW events during HW periods. Synergies between HWs and the UHI amplify both the spatial and temporal coverage of high-temperature events, which in turn exposes urban residents to additional heat stress and seriously threatens their health. The findings have important implications for HWs and UHII forecasts, as well as for scientific guidance on decision-making to improve the thermal environment and to adjust the energy structure.
Highlights
Economies around the world are developing rapidly with global economic integration
With the acceleration of urbanization, urban and suburban stations have experienced varying degrees of warming, which has led to a more extensive urban heat island (UHI) effect, and caused more heat wave (HW) events with long durations in urban areas compared to rural areas (Tan et al, 2010; see Supplementary Figure S1)
Due to the unique underlying city canopy-layer structure with good thermal conductivity and large heat capacity (Stewart and Oke, 2012; Ren, 2015; Wang L. et al, 2020), the increase in net heat flux was more over urban areas than rural areas (i.e., Δ QN, Rural
Summary
Economies around the world are developing rapidly with global economic integration. the processes of urbanization and industrialization are accelerating year by year. Due to increases in population density, immense changes in land use, increases in anthropogenic heat emissions, and reduced green space have led to strong urban heat island (UHI) effects (Yang et al, 2016; Li et al, 2020). This phenomenon, manifested by enhanced air/surface temperature in urban areas compared to their rural surrounding areas (Oke and Maxwell, 1975; Roth, 2007), is one of the key characteristics of urban climates. Many studies have pointed out that, usually, the UHII in Beijing is stronger in winter and weaker in summer (Xie et al, 2006; Yang et al, 2013); while in terms of daily variation, it tends to be stronger at night than during the day (Ren et al, 2007; Huang and Lu, 2018)
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